Some horses will spontaneously become “nippers”, and that can be very dangerous. Horses can actually become addicted to meat and be downright dangerous.
Usually it begins with someone who for some reason puts some hamburger in the horses food. In short order, unless it is given a regular ration of meat, the horse can become belligerent and aggressive. Some take to killing and eating cats that stray into their pen, and will most definitely bite people with the intent to bite off part of them.
Was bitten in the face by a gelding - just hanging out between two stalls - he had a really evil look in his eyes and just attacked me. I think it was an alpha male thing.
After that incident the other kids were scared of him - but I made sure not to be fearful and would crawl under his belly before putting his saddle on. He never bit anyone again - so I don’t know for certain why he was so vicious that one time.